Poly(vinylamines) are polymers which can be prepared over a broad range of molecular weights. Depending upon their average molecular weight, such polymers find various uses in the preparation of dyes, pharmaceuticals, flocculation agents and as viscosifiers in papermaking and enhanced recovery of oil. Because vinylamines are too unstable to be polymerized, these polymers are prepared by hydrolysis of poly(N-vinylamides) such as poly(N-vinylacetamide). The monomer for this polymer is made by the reaction of acetamide and acetaldehyde to form ethylidene bis-acetamide which is then pyrolyzed to the N-vinylacetamide. This reaction is described in Dawson, et. al. JACS, 98, pg. 5996-6000 (1976). An improvement in the basic process is described as a reaction between acetamide and acetaldehyde over a sulfuric acid catalyst, rather than perchloric acid, in order to form ethylidene bis-acetamide which was then pyrolyzed to form N-vinylacetamide. The N-vinylacetamide was polymerized and the polymer subjected to hydrolysis to form poly(vinylamine hydrochloride) which was used in preparing polymeric azo dyes.
U.S. Pat. No. 4,018,826 (1977) also discloses a method of making poly(vinylamine) by hydrolyzing poly(N-vinylacetamide) which has been formed by thermally cracking ethylidene bis-acetamide prepared by reacting acetaldehyde and acetamide using an aqueous mineral acid catalyst.
It was known that a similar reaction could take place between formamide and acetaldehyde in an aqueous solution of hydrochloric acid as described in "Journal of Organic Chemistry", Volume 36, No. 2, pg. 351 (1971), which describes a method for making 1,1-bis(formamido)ethane, another name for ethylidene bis-formamide.
U.S. Pat. No. 4,490,557 (1984) discloses the preparation of ethylidene bis-formamide from acetaldehyde and formamide using an acidic catalyst and an ammonia scavenger, such as acetic anhydride. Wiped film evaporation is used to recover the ethylidene bis-formamide which can be cracked to form N-vinylformamide, a monomer useful in preparing poly(N-vinylformamide) which can then be hydrolyzed to poly(vinylamines), useful in making dyes and pharmaceuticals. The acidic catalysts which are disclosed include acidic ion exchange resins, of which several examples are given including the preferred operating example. Alternatively, mineral acids, such as sulfuric or hydrochloric acid, or lower alkanoic acids, such as formic or acetic acids, can be used when added in catalytically effective amounts. Although such amounts are stated to cover a broad range from about 0.001 to 1 mole of acid catalyst per mole of formamide, the preferred catalytically effective amount is 0.002 to 0.1 mole of catalyst per mole of formamide and there is no suggestion of any advantage in using solvent quantities of any of the acids in the reaction mixture.
Because of the difficulty in recovering polymerization grade monomers by the above described routes, others have sought to prepare N-vinylcarboxylic acid amides by different routes. U.S. Pat. No. 4,322,271 (1982) discloses that N-vinyl-N-alkyl-carboxylic acid amides can be obtained by removing an alcohol from N-.alpha.-alkoxyethyl-N-alkyl-carboxylic acid amides which have been made by prior alkylation and alkoxylation steps from N-ethyl-carboxylic acid amides.
Sommerville, et. al., ACS, Polymer Preprints, (1983) 24, 12-13, discloses preparing N-vinylacetamide from acetamide and the acetaldehyde dimethyl acetal. This process requires large excesses of the acetal, for example mole ratios of about 20 moles of acetal per mole of acetamide, in order to achieve practical yields and purities and is reported to fail in the corresponding reaction with formamide.
U.S. Pat. No. 4,567,300 (1986) discloses, on the other hand, reacting formamide with acetaldehyde over a basic catalyst to form N-(.alpha.-hydroxyethyl)-formamide instead of ethylidene bis-formamide. This process is unattractive because it requires two discrete steps, plus the handling of a solid intermediate and the disposal of salts.
U.S. Pat. No. 4,670,591 (1987) describes the synthesis of N-alkoxyethyl formamide from vinylether and formamide. While this process is said to be effective with either an acidic or basic catalyst, the vinylethers are very expensive starting materials.
German Patent DE3443463 (1986) describes making N-vinylformamide using 1-cyanoethyl formamide. This process has the disadvantage of generating hydrogen cyanide which is very toxic.
The above processes as routes to the manufacture of poly(vinylamines) all have the disadvantage of difficult catalyst removal, toxic byproduct formation, low conversions or catalyst deactivation. A commercially practicable process which does not have these disadvantages has yet to be developed. Ethylidene bis-formamide is still an attractive intermediate for the synthesis of N-vinylformamide as this product is stable and can be efficiently cracked thermally to form a 1:1 mixture of N-vinylformamide and formamide. Such a mixture can be purified by distillation as described in U.S. Pat. No. 4,578,515.
The preparation of ethylidene bis-formamide using strong acid ion exchange resins as disclosed in U.S. Pat. No. 4,490,557, has the disadvantage that the strong acid catalyst residues must be removed from the ethylidene bis-formamide product prior to purification and cracking. Otherwise unwanted side reactions and loss of N-vinylformamide due to acid catalyzed degradation in the cracking step are observed. The use of the solid polymer acid resins, on the other hand, allows the removal of salts and catalyst as a solid from the liquid product. Unfortunately, however, in the synthesis of ethylidene bis-formamide the catalyst activity declines rapidly during the reaction, giving poor conversions. This is caused by the hydrolysis of formamide and neutralization of the catalyst with ammonia. Since water is produced in the synthesis of ethylidene bis-formamide and high levels of formamide are required to drive the synthesis reaction, it is not feasible to suppress the formation of ammonia using prior art technology. The result is poor conversions and impure product, probably arising from unwanted acetaldehyde self-condensation reactions. It is highly desirable, therefore, to find a way of improving the yields of ethylidene bis-formamide in such reactions and reducing the loss of formamide by hydrolysis.